So this is kind of a weird one. I have a 2003 Expedition with 5.4L. It has about 215,000km on it.

When I first start my truck up, it operates and idles normally. Then I go for a drive. Sometimes it might drive more or less just fine, to destination and back with no issues. But other times you feel a bit of a rumble going on, like it's missing. When you hit the gas it feels slightly uneven.

Then you come to a stop. Suddenly the RPM drops to below 1000 (even below 500), and gets so quiet it's like it's only half functional. You hit the gas to prevent it from stalling but there is absolutely no response at all. It just stays sub-1000 RPM, does not respond to anything you do, getting really quiet like its slowly running out of gas, and then eventually dies. Once in a while it will not die and suddenly rev up (I guess the computer catching and correcting), but usually death.

Then you try to restart it. It takes about 6 tries before it catches again. Then it's fine for another half hour, albeit with that slight rumble I mentioned.

I thought it might be IAC, but from what I've read, stomping the accelerator ought to pull it out of that 'near death' idle. Nope. Another possibility and one I've run into before was bad spark plug coil. But with that I've just experienced rough idle, weird surging when you accelerate. Not the 'near death' idle.

I read another forum post where the user mentioned a wire that ran on top of AC lines and over time wore through the insulation and eventually would be shorting on the metal, causing the whole system to crash randomly. I did inspect this and did find a similar situation, but I've corrected it by fixing the wire and taping it up, to no avail.

I also replaced the fuel filter. Before all this started my wife ran the truck down to vapors (she does this often with our vehicles). So I kind of suspected maybe something got sucked up from the bottom of the tank and tried going after various failure points like the filter, running some FI cleaner through, etc. But no change.

It's frustrating because the issue is totally random, and of course, never happens when it's at the shop.

Many thanks in advance for any suggestions!

 

Check for codes

 

It isn't throwing any codes. Sorry, I should have mentioned that.

 

Original fuel pump ? Can you take a fuel pressure reading there at the fuel rail? Should see 45-55 psi in the rail at all times. The schrader valve is located on the fuel rail (passenger side US market...).

 

Definitely check fuel pressure. If they didn't move the schrader valve too far from where it is on a '97 (back of rail on passenger side), it's pretty easy to hook up and test. Just go down to your local parts house that loans tools. They'll have a fuel pressure gauge.

 

This sort of sounds like a bad fuel pump relay. I had to replace mine a couple years ago. Worth checking out.
https://www.ford-trucks.com/forums/1...dom-stall.html

 

The IAC is only a factor at idle, once you touch the gas pedal the IAC is out of the equation, so it has nothing to do with the rough running. The IAC also will not prevent the RPMs from climbing when you press the gas pedal. I would do as others have already said and test the fuel pressure.